When a public mobile communications device such as a cell phone places a 911 or other emergency call, a facility known as a Public Safety Answering Point, or PSAP, responds to the call. The PSAP determine the nature of the emergency and engages the appropriate local public safety service, e.g., police, fire etc. Each PSAP serves a geographic area defined by a polygon on the earth's surface, and a state or province may include many PSAPs. Thus, when an emergency call is placed by a mobile user, the appropriate PSAP must be quickly determined so that the necessary emergency services may be timely called to action.
The mobile communications device may know its current location from the network that services it, from a radio navigation service such as GPS, or by other means. The mobile device could simply transmit its location to its service provider, and request the provider to determine the appropriate PSAP. In some scenarios, however, it may be undesirable or hazardous for the mobile device to reveal its exact location to unauthorized or untrusted parties during an emergency call. For example, it may be undesirable for a military person making a medical emergency phone call during combat to disclose location information to network servers accessible by hostile parties.
Alternatively, the mobile device could determine what PSAP serves its current location by determining which polygon it is in when the call is placed. This can readily be done when the mobile device remains within or near its normal operation location. For example, by recognizing the cell tower in use, the service provider (SP) knows that the mobile device is within the corresponding cell boundary, and may be able to determine a corresponding polygon and PSAP. Consider, however, a scenario wherein the mobile device is registered to an SP in, e.g., England, and the device is currently served by a network somewhere in the United States, e.g., in San Jose, Calif. It is unlikely that the home SP in England will know the polygon structure for California.
The National Emergency Numbering Association (NENA) and the Internet Emergency Task Force (IETF) are studying the use of Domain Name Servers (DNS) to allow mobile devices to learn PSAP polygons. The mobile device sends a DNS query, and a complete set of hierarchical polygon information is sent to the mobile device. Such information may include polygons for many or all countries in the world. A complete definition of the structure of all of these polygons must be downloaded to the mobile device. The mobile device first determines which of the largest polygons it is in, which might correspond to determining the country. The mobile device then retrieves polygon information regarding all the states or provinces within the country, and determines which state or province it is in. The process is repeated to the county and finally to the PSAP level, after which the mobile device finally contacts the appropriate PSAP. As may be appreciated, considerable information must be transferred. Since polygons may have many vertices, and since the transmission speed may be relatively low, an inordinate amount of time may be required to transfer the necessary information. Meanwhile, experience has shown that if an emergency call does not go through within several seconds, the user will often abandon the call and try again.
From the above, there is a need for an improved system and method to enable a mobile communication device making an emergency call to efficiently determine the appropriate PSAP, without divulging detailed location information.